Ears 
Remarkably helpful 
Preferably your own ears as most people prefer to hang on to their own.
BTW, this is actually an excellent concept for a thread.
I am assuming you are male, since most are in this hobby.
I would suggest a female set of ears, your better half, or a younger female. Females have more sensitive hearing. (Try saying something in the next room, and I'll bet she can clearly hear you, especially in an argument.)
Younger because most still can hear whole bandwidth of the audio spectrum.
As per trained ears, you can probably train them of the sonic nuances of music audiophools listen to.
Hope this helps
